NOTE: As their name suggests hints are not exactly Project Gotham Racing 4 cheats, and are usually just simple (but very useful) tips on how you can unlock secret level, features, find hidden areas and such. Hopefully these will make playing Project Gotham Racing 4 even more entertaining.

Achievements
Complete the following achievements to unlock Xbox Live Gamerscore points.

1-2 Finish (10): Complete a Ranked Match Team Championship with your team taking the top two positions.
3CZV657 (5): During any stormy race, be travelling at over 88mph in a Delorean when lightning flashes.
5-Star Racer (20): Earn 5 Kudos Stars.
8th Wonder of the World (20): Complete a lap of the Michelin Test Track 'figure of 8' while sliding.
Arcade Chapter Complete (30): Complete any chapter in Arcade mode.
Back it Up! (10): In split-screen Street Race, finish 1st and 2nd in reverse gear against at least one AI opponent.
Beat Bizarre (10): Defer a PGR 4 team member on Xbox LIVE, or defeat anyone who's bested a PGR 4 team member.
Bibendum (10): Play all 3 Test Track disciplines and set a score in each one.
Buy an Achievement (20): Buy the K1,000,000 gamer picture to earn this Achievement.
Catch 'em All (5): Swap paint with every opponent in an eight vehicle race.
Champion (10): Win your first Championship in either Single Player Career or Ranked Match play.
Dominator (20): Win all three Single Player Career Majors.
Endo King (20): Earn 100 Kudos by performing an Endo with a motorcycle.
First Timer (10): Complete your first Ranked Match Championship.
Giant Killer (20): Win against a human opponent with Rank 1 single player status on Xbox LIVE.
Gold Double (5): Earn at least a Gold medal with both a bike and a car in any Arcade event.
Gold Gotham Arcader (10): Earn a Gold medal in every Arcade event.
Hero (20): Become a National Champion in Single Player Career mode.
Hotshot (70): Reach the rank of Hotshot in Single Player Career mode.
I Love PGR (5): Buy everything in the PGR shop or complete PGR3 and get to Rank #1.
Major Winner (40): Win a Major in Single Player Career mode.
Master (40): Reach the rank of Master in Single Player Career mode.
Mind Your Manners (25): Win a Street Race in Arcade or Gotham Career mode without making contact with walls.
Motorcycle Ace (25): Win a Single Player Career Championship in a motorcycle.
Numero Uno (50): Reach Rank Number One on the Single Player Career Drivers' Leaderboard.
PGR On Demand Photo (5): Upload a photo and vote on a photo in PGR On Demand.
PGR On Demand Video (5): Upload a replay and vote on a replay using PGR On Demand.
Platinum Double (5): Earn at least a Platinum medal with both a bike and a car in any Arcade event.
Platinum Gotham Arcader (10): Earn a Platinum medal in every Arcade event.
Play It Again Sam (20): Improve on a medal earned in any arcade event.
Professional (70): Reach the rank of Professional in Single Player Career mode.
Rubbin' is Racin' (20): Knock off the wing mirrors on all your opponents' vehicles in a Single Player Street Race event.
Seat of your Pants (25): Complete a Hot Lap event in Arcade mode without touching your break.
Secret Agent (5): Select the Aston Martin DBS. Do a race in Las Vegas. Look for a "Casino Royale" building, then park at the front.
Show Jump (10): Get eight vehicles in the air at the same time.
Showboater (20): Earn 9000 Kudos in a single race.
Silver Double (30): Earn at least Silver medal with both a bike and a car in any Arcade event.
Silver Gotham Arcader (40): Earn a Silver medal in every Arcade event.
Strike a Pose (10): Get two bikes facing each other, 15 metres apart, with each performing an impressive endo.
Stuntman (5): Perform your first stunt on a bike.
Superstar (40): Earn 5 Kudos Stars three times in a race.
Team Domination (20): Complete a Ranked Match Team Championship with your team taking up the top four positions.
Team Leader (10): Earn the most points for your team in a Ranked Match Team Championship.
The Unicyclist (20): Perform a long Wheelie on a motorcycle.
Tonight Make Me Unstoppable (5): Get second place during a race while listening to the song Bloc Party - Prayer.
Weather Master (25): Win races in rain, fog, and snow in either Arcade or Single Player Career modes.
Whitewash (50): Win every event in a Single Player or Ranked Match Championship.
Winner (5): Win your first event in either Arcade or Single Player Career Mode.
Winning Streak (30): Win 3 race events back to back in either Arcade or Single Player Career Mode.
You Won't Believe Your Eyes! (5): Create a 3D image using Photo Mode.

DLC Achievements
Complete each achievement to get the allotted gamerscore.

01 Yee Haw (10): Puzzle #4
Arcade Challenge (50): Complete the Arcade Challenge chapter (at various difficulties)
Are You the One? (10): Puzzle #5
Cheater (5): Activate one of the cheats in the cheat menu
Download King (60): Complete the DLC Challenge chapter (at various difficulties)
Make the Grade (10): Try to qualify for a tournament
Open Cat Champion (15): Be a cat on the winning team in a Cat and Mouse Open Event
Open Mouse Master (15): Be a mouse on the winning team in a Cat and Mouse Open Event
Platinum Plus (60): Get all the platinums in the Platinum Plus Challenge
Tourist Kudos King (15): Earn more kudos than anyone else in a Tourist event